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Arenes booklet
Arenes are aromatic hydrocarbons. The term "aromatic" originally referred to their pleasant smells, but now
implies a particular sort of delocalised bonding.
The simplest of them is benzene itself, C6H6. The next simplest is methylbenzene (old name: toluene) which
has one of the hydrogen atoms attached to the ring replaced by a group - C6H5CH3.
Important facts about benzene are:
➢ Benzene, C6H6, is a molecule containing a ring of six carbon atoms each with a
hydrogen atom attached. The six carbon atoms form a perfectly regular . All the
carbon-carbon bonds have exactly the same lengths - somewhere between single and double bonds.
➢ There are delocalised electrons above and below the plane of the ring.
➢ The presence of the delocalised electrons makes benzene very .
➢ Benzene resists addition reactions because that would involve breaking
the delocalisation and losing that stability.
➢ Benzene is usually represented using a formula, where the
circle represents the electrons, and each corner of the hexagon
has a carbon atom with a hydrogen atom attached.
Draw skeletal formulae for the following benzene derivatives:
methylbenzene 3-methylchlorobenzene
2-hydroxybenzoic acid 2-chlorophenylamine
(phenyl can be used instead of benzene for amines)
, 2
The stability of benzene
The hypothetical Kekulé structure shown below (named cyclohexa- -triene) has a molecular
formula C6H6 but X-ray analysis showed that the carbon-carbon bonds in the ‘real’ benzene are all the same
length and are intermediate between a single and double bond.
Three carbon−carbon bonds are labelled on the structures shown.
These bonds are of different lengths.
Write the letters w, x and y in order of increasing bond length
If you do a thermochemistry calculation based on the hypothetical Kekulé structure you get an answer
which is wrong by about 150 kJ mol-1. An example of this is for the enthalpy change of hydrogenation.
In this reaction hydrogen atoms add on across the carbon-carbon double bond.
Use the enthalpy level diagram on the next page to find the enthalpy changes.
The enthalpy change of hydrogenation of cyclohexene (in the presence of a finely divided nickel catalyst at a
temperature of about 150°C) is kJmol-1.
Based on this data, you would predict that the enthalpy change of hydrogenation of the hypothetical Kekulé
structure would be about kJ mol-1.
The actual enthalpy change of hydrogenation of the ‘real’ benzene is kJ mol-1 .
There is a discrepancy of kJ between the enthalpy change of hydrogenation of the hypothetical
Kekulé structure and ‘real’ benzene. The ‘real’ benzene has less energy than the hypothetical Kekulé
structure and is therefore more stable.
Arenes booklet
Arenes are aromatic hydrocarbons. The term "aromatic" originally referred to their pleasant smells, but now
implies a particular sort of delocalised bonding.
The simplest of them is benzene itself, C6H6. The next simplest is methylbenzene (old name: toluene) which
has one of the hydrogen atoms attached to the ring replaced by a group - C6H5CH3.
Important facts about benzene are:
➢ Benzene, C6H6, is a molecule containing a ring of six carbon atoms each with a
hydrogen atom attached. The six carbon atoms form a perfectly regular . All the
carbon-carbon bonds have exactly the same lengths - somewhere between single and double bonds.
➢ There are delocalised electrons above and below the plane of the ring.
➢ The presence of the delocalised electrons makes benzene very .
➢ Benzene resists addition reactions because that would involve breaking
the delocalisation and losing that stability.
➢ Benzene is usually represented using a formula, where the
circle represents the electrons, and each corner of the hexagon
has a carbon atom with a hydrogen atom attached.
Draw skeletal formulae for the following benzene derivatives:
methylbenzene 3-methylchlorobenzene
2-hydroxybenzoic acid 2-chlorophenylamine
(phenyl can be used instead of benzene for amines)
, 2
The stability of benzene
The hypothetical Kekulé structure shown below (named cyclohexa- -triene) has a molecular
formula C6H6 but X-ray analysis showed that the carbon-carbon bonds in the ‘real’ benzene are all the same
length and are intermediate between a single and double bond.
Three carbon−carbon bonds are labelled on the structures shown.
These bonds are of different lengths.
Write the letters w, x and y in order of increasing bond length
If you do a thermochemistry calculation based on the hypothetical Kekulé structure you get an answer
which is wrong by about 150 kJ mol-1. An example of this is for the enthalpy change of hydrogenation.
In this reaction hydrogen atoms add on across the carbon-carbon double bond.
Use the enthalpy level diagram on the next page to find the enthalpy changes.
The enthalpy change of hydrogenation of cyclohexene (in the presence of a finely divided nickel catalyst at a
temperature of about 150°C) is kJmol-1.
Based on this data, you would predict that the enthalpy change of hydrogenation of the hypothetical Kekulé
structure would be about kJ mol-1.
The actual enthalpy change of hydrogenation of the ‘real’ benzene is kJ mol-1 .
There is a discrepancy of kJ between the enthalpy change of hydrogenation of the hypothetical
Kekulé structure and ‘real’ benzene. The ‘real’ benzene has less energy than the hypothetical Kekulé
structure and is therefore more stable.
